The present invention relates generally to electronic circuits, and more particularly to field-effect transistors with applications to displays and sensors.
Displays and sensors, such as touchscreens in smartphones, are part of interfaces for interacting with many electronic devices. As an example, an OLED (organic light-emitting diode) display may include an array of TFT (thin-film transistor) pixel circuits, where each pixel circuit includes an OLED controllable by one or more select lines and one or more data lines. In a typical pixel circuit, a transistor, such as a TFT FET (field-effect transistor), when switched on provides drive current to its associated OLED. Such a transistor may be termed a driver TFT or driver transistor.
In some applications, such as high-resolution displays, it is desirable to reduce the dimensions of the pixels. In general, as the dimensions of a pixel (and therefore the dimensions of its OLED) are reduced, less drive current is needed for the OLED, and therefore to reduce power dissipation and improve performance it can be desirable for the driver transistor to operate in its subthreshold region. Furthermore, in some applications it is desirable to control the brightness of an OLED with a high precision, e.g., to generate a large number of distinct brightness levels. This may be achieved by controlling the gate-to-source voltage of the driver TFT so as to vary the drain current of the driver TFT as it is operating in its subthreshold region. A driver TFT with a relatively large subthreshold swing can be desirable because less demand is placed upon the devices and circuits used to accurately control its gate-to-source voltage.